This invention relates generally to sensors and more particularly relates to a pressure and/or temperature sensor having exceptional stability up to 200xc2x0 C. and effectively operable up to 700xc2x0 C. The pressure sensor of the present invention operates without fluid fill and has no exterior exposed metallic components. The pressure sensor includes a non-porous, impermeable surface that may be positioned in direct contact with fluids in an ultra-pure environment. In one embodiment of the present invention, the non-porous surface is comprised of a layer of single crystal sapphire that is impervious to chemical attack. In this manner, chemicals or contaminants cannot be extracted over time from the sensor into a process stream. Without limitation, the pressure sensor of the present invention is suitable for use in a chemically inert pressure transducer module or flow meter for sensing pressures in process fluids and may be molded directly into the high temperature plastic housing of the same.
Pressure sensors have been utilized in various applications to measure either gauge pressure or absolute pressure. Many of these applications involve the measurement of pressure in unfavorable environments. The pressure sensor may be of a capacitive type or piezoresistive type. For example, an alumina ceramic capacitive sensor may comprise a thin, generally compliant ceramic sheet having an insulating spacer ring sandwiched between a thicker, non-compliant ceramic sheet. The first thin ceramic sheet or diaphragm is approximately 0.005 to 0.050 inches in thickness with a typical thickness of 0.020 inches. The thicker ceramic sheet has a thickness range between 0.100 to 0.200 inches. Those skilled in the art will appreciate that the thickness of the diaphragm is preferably dependent upon the diameter of the diaphragm. The spacer may be constructed of a suitable polymer. The apposed faces of ceramic disks are metalized by metals such as gold, nickel or chrome to create plates of a capacitor. A similar capacitive pressure transducer is described by Bell et al. in U.S. Pat. No. 4,177,496 (the ""496 patent). Other capacitive pressure transducers similar to that described in the ""496 patent are available and known in the art. A piezoresistive sensor typically utilizes a Wheatstone bridge, measuring changes in voltage and correlating the voltage changes to changes in sensed pressure. Either of these pressure sensor types may be utilized to measure the pressure of fluids in ultra-pure environments, however, there is a need for a non-contaminating pressure sensor.
Ultra pure processing of sensitive materials typically requires the use of caustic fluids. The susceptibility to contamination of the sensitive materials during the manufacturing process is a significant problem faced by manufacturers. Various manufacturing systems have been designed to reduce the contamination of the sensitive materials by foreign particles, ionic contaminants, and vapors generated during the manufacturing process. The processing of the sensitive materials often involves direct contact with caustic fluids. Hence, it is critical to deliver the caustic fluids to the processing site in an uncontaminated state and without foreign particulate. Various components of the processing equipment are commonly designed to reduce the amount of particulate generated and ions dissolved into the process fluids, and to isolate the processing chemicals from contaminating influences.
The processing equipment typically includes liquid transporting systems that carry the caustic chemicals from supply tanks through pumping and regulating stations and through the processing equipment itself. The liquid chemical transport systems, which includes pipes, pumps, tubing, monitoring devices, sensing devices, valves, fittings and related devices, are frequently made of plastics resistant to the deteriorating effects of the caustic chemicals. Metals, which are conventionally used in such monitoring devices, cannot reliably stand up to the corrosive environment for long periods of time. Hence, the monitoring and sensing devices must incorporate substitute materials or remain isolated from the caustic fluids.
While the processes must be very clean they often involve chemicals that are very aggressive. These could include for example harsh acids, bases, and solvents. The semiconductor industry has recently introduced processes which make use of aggressive abrasives. Both the process equipment and the monitoring instrumentation must be impervious to the mechanical action of these abrasives.
Further, high reliability of process equipment instrumentation is a must. Shutting down a semiconductor or pharmaceutical line for any reason is costly. In the past, pressure transducers have commonly employed fill fluids to transmit pressure from the process to the sensor itself. The fill fluids are separated from the process by an isolator diaphragm of one sort or another. Failure of this isolator diaphragm and subsequent loss of fill fluid into the process can cause loss of product and require system cleaning before restarting operations. Eliminating the isolator diaphragm and fill fluid from the design is advantageous.
Also, the processing equipment commonly used in semiconductor manufacturing has one or more monitoring, valving, and sensing devices. These devices are typically connected in a closed loop feedback relationship and are used in monitoring and controlling the equipment. These monitoring and sensing devices must also be designed to eliminate any contamination that might be introduced. The sensing devices may include pressure transducer modules and flow meters having pressure sensors. It may be desirable to have a portion of the pressure sensor of the pressure transducer or flow meter in direct contact with the caustic fluids. Thus, the surfaces of the pressure sensor in direct contact with the caustic fluids should be non-contaminating. It has been found that porous materials allow the ingress and egress of caustic fluids through such materials. For example, ceramic materials are bound together with various glass like materials which themselves are easily attacked by the more aggressive corrosive materials. Hence, it is preferable that those portions of the pressure sensor in direct contact with caustic fluids be made of non-porous materials.
U.S. Pat. No. 4,774,843 issued to Ghiselin et al. describes a strain gauge having a single crystal sapphire diaphragm adhered to an aluminum oxide base. Ghiselin et al. indicates that the sapphire is adhered by means of a glass bonding material, epoxy or other adherent methods. Ghiselin et al. does not provide a further description of the glass bonding material or how the glass bond adheres to the sapphire and aluminum oxide base. However. Ghiselin describes the glass bond as a low strength material that separates at strain points. Ghiselin describes a change in geometry to reduce the strain point and thereby avoid the deficiencies of the low strength of the glass. U.S. Pat. No. 5,954,900 issued to Hegner et al. describes problems with using a glass to bond to an aluminum oxide ceramic part. Hegner et al. describes the use of alumina as the joining material to alumina ceramic. The devices described by Hegner et al. and Ghiselin et al. are believed to be limited to effective operable temperatures below 400xc2x0 C. Thus, the reliability of the sensors described by Hegner et al. and Ghiselin et al. decreases as temperatures exceed 400xc2x0 C. The caustic fluids of the processing equipment may often exceed 400xc2x0 C. Hence, there is a need for a pressure sensor having a non-porous surface that is bonded to the base with a high strength bond, wherein the bond between the non-porous material and the base is stable at temperatures in excess of 400xc2x0 C.
It has also been found that Electromagnetic and Radio Frequency Interference (EMI and RFI respectively) degrade the performance of piezoresistive sensors. A conductive shielding layer cannot be positioned directly between a silicon layer (on which the Wheatstone bridge is formed) and the sapphire because of the epitaxial construction of silicon on sapphire. A conductive shielding layer on the outside of the sapphire is not preferred when the outside of the sapphire is positioned in contact with the caustic fluids. Hence, a need exists for a non-contaminating pressure sensor that blocks the EMI and RFI from affecting the sensing element formed on a non-exposed surface of the pressure sensor. The present invention meets these and other needs that will become apparent from a review of the description of the present invention.
The present invention provides for a pressure sensor that includes a non-porous outer surface. The non-porous surface is characterized by a low diffusivity and low surface adsorption. In the preferred embodiment, the pressure sensor includes a backing plate, a non-porous diaphragm, a sensing element adjacent an inner surface of the diaphragm, and a glass layer of a high strength material that is bonded by glassing to the backing plate and the non-porous diaphragm. The backing plate provides rigidity to the structure. The rigidity of the backing plate resists stresses transmitted from the housing (not shown) to the sensing elements on the sensor diaphragm. Although the backing plate is not in direct contact with the process medium it is required to be mechanically stable and amenable to high temperature processes. The thermal expansion rate of the backing plate should approximate closely that of the sensing diaphragm. While it is possible to compensate for thermal effects, a large mismatch will produce stresses during manufacture that may cause the bond between the two pieces to yield over time. Those skilled in the art will appreciate that the non-porous diaphragm may include a Wheatstone bridge or a conductive layer formed thereon as part of a piezoresistive or capacitive type sensor respectively.
Without limitation, in the preferred embodiment, a silicon layer is formed on an inner surface of the non-porous diaphragm, wherein a strain gage such as a Wheatstone bridge is formed thereon. The backing plate includes apertures extending therethrough, the apertures being adapted for receiving electrical leads coupled to the sensing element. A change in pressure near the non-porous diaphragm is detectable by the sensing element. An increase and decrease of pressure against the diaphragm causes deflection of the diaphragm which in turn changes the resistances of the strain gage. The changes in resistance is correlated with the pressure adjacent the diaphragm.
Without limitation, the non-porous diaphragm is preferably comprised of a chemically inert material such as sapphire. The glass layer between the sapphire and the backing plate is preferably made of high bond strength borosilicate glass or other glass of suitable known construction having a high bond strength and melt temperature above 700xc2x0 C. and preferably above 1000xc2x0 C. The amount that the diaphragm flexes is controlled by the thickness and diameter of the glass layer. The glass layer may have a thickness ranging between 0.002 and 0.030 inches with 0.010 inches being preferred and an outside diameter ranging from 0.100 to 2.0 inches with 0.700 inches being preferred. The active sensing area of the diaphragm may range from 0.050 to 2.0 inches with 0.400 inches being preferred. Those skilled in the art will appreciate that the range of thickness and diameter of the diaphragm should not be construed as limiting, but that the thickness and diameter in certain applications may be further reduced or increased as desired. In this manner, when the non-porous diaphragm flexes to the maximum flexure, a portion of the inner surface of the diaphragm engages an inner surface of the backing plate. Those skilled in the art will appreciate that the backing plate and non-porous diaphragm are constructed of materials having similar thermal expansion rates to avoid unnecessary stress through a wide range of temperatures. As described below in greater detail the pressure sensor may be constructed such that the sensing element may detect an absolute pressure or gage pressure.
The pressure sensor may further include a silicon nitride layer and a metalization or conductive layer positioned between the silicon layer and the backing plate (see FIG. 11). In this manner the silicon nitride layer acts as an electrical insulator and the metalization layer blocks EMI/RFI from affecting the sensing element 20. The pressure sensor may further include a coating, gasket or seal adjacent to at least a portion of an outer edge of the layers of the non-porous diaphragm, silicon nitride layer, metalization layer and the backing plate. Without limitation, acid resistant epoxy or corrosion resistant polymers such as PTFE (polytetrafluroethylene), PVDF (Polyvinylidenefluoride), PEEK (polyetheretherketone), urethane, or parylene may be utilized, wherein an acid resistant epoxy is preferred.
The pressure sensor includes bond pads formed on the diaphragm between the glass layer and the non-porous diaphragm. Without limitation, the preferred embodiment of the bond pads comprise a titanium layer and a diffusion barrier. The doped silicon thin film interconnects the bond pads in a known suitable manner to form the Wheatstone bridge. A window is formed in the glass layer and backing plate, thereby providing access to bond pads. Electrical leads extend through the windows formed in the glass layer and backing plate and the electrical leads are brazed to the bond pads. The electrical leads are brazed to the bond pads and the glass layer is glassed to the diaphragm and backing plate.
In an alternate embodiment the diaphragm and sensing element is modified to create a capacitance rather than a piezoresistive sensor. The thin sensing diaphragm, which flexes when pressure is applied, has a capacitive plate formed on the inner surface of the sensing diaphragm and another capacitive plate is formed on the inner surface of the backing plate. One electrical lead is connected to the capacitive plate formed on the inner surface of the sensing diaphragm and another lead is electrically coupled to the inner surface of the backing plate. As the spacing between the diaphragm and the plate vary with pressure the capacitance of the plates changes. This variation in capacitance is detected by an electrically connected sensing element of known suitable construction.
The advantages of the present invention will become readily apparent to those skilled in the art from a review of the following detailed description of the preferred embodiment especially when considered in conjunction with the claims and accompanying drawings in which like numerals in the several views refer to corresponding parts.